Superinfection to the rescue

Vaccines have successfully eradicated once widespread diseases such as smallpox and polio.  However, despite decades of human immunodefiency virus (HIV) vaccine research, the World Health Organization reports that 36.7 million people worldwide were living with HIV at the end of 2016.  Vaccines work by mimicking natural infections, during which the body produces antibodies that eliminate the virus. Unlike smallpox or polio, HIV is a sly beast, and evades the immune system so the body is unable to launch an effective antibody attack.  

Nevertheless, the HIV neutralizing antibody (nAb) response following natural infection is a promising approach to generate effective responses to HIV vaccination. Experiments in animal models have shown that administering broadly neutralizing antibodies (bnAb) is sufficient to protect against HIV infection. Clinical studies of bnAb have focused on individuals with a plasma signature indicative of a dominant epitope-specific response; however, less is known regarding individuals with a polyclonal response.  Nevertheless, these responses are extremely hard to study because only a fraction of chronically infected individuals develop antibodies that can neutralize diverse HIV strains. Importantly, for a vaccine to provide good coverage across diverse viruses and to prevent reoccurrence of the virus after infection, the ability to elicit a polyclonal response would be imperative.

Dr. Julie Overbaugh and members of her laboratory in the Human Biology Division have been actively studying bnAb responses in HIV infected individuals over the last few years.  They first showed that individuals infected with two distinct HIV strains from different partners, a process known as superinfection, have broader nAb responses compared to singly infected individuals.  The prevalence of superinfection is about half that of first infections. This greater breadth in nAb responses may be attributed to antigenic diversity generated by the second, genetically distinct virus. However, a subsequent study of 21 superinfected individuals failed to define plasma epitope targets. To further investigate the mechanism of why superinfected individuals have greater breadth in the nAb responses, Dr. Overbaugh and her lab characterized the monoclonal antibody repertoire of a superinfected individual who developed a broad and potent plasma nAb response consistent with a polyclonal repertoire. The results of their study, published in Cell Reports, showed that the breadth in responses can be attributed to unique Ab lineages that specifically target either the initial or the subsequent virus.

To determine the target of the superinfected plasma nAb, memory B cells (mBCs) from the superinfected individual, referred to as QA013, were obtained. The supernatants from mBCs were tested against two HIV variants neutralized by QA013 plasma in a culture-based approach to identify HIV-specific mBCs. Six nAbs from four lineages were identified as HIV-specific, and mediated neutralizing activity against 10 out of 19 viruses, representing a breadth of 53%. These nAbs were also screened against an additional virus panel, where the authors observed that these nAbs neutralized a broader panel of viruses than any lineage tested individually, suggesting a polyclonal nAb response. In particular, nAb QA013.2 had 41% breadth, and showed potent, cross-clade activity. These antibodies typically target epitopes on the HIV envelope. Specifically, the authors reported that nAb QA013.2 targeted a high-mannose patch of gp120 centered around the glycan at position 332 (N332), also known as the N332 glycan supersite. This HIV envelope glycan shield is used by the virus as a defense mechanism to evade immune recognition; N332-directed responses are common following HIV infection and are often targets of vaccine efforts.  Interestingly, mechanisms of viral escape involve the N332 mutation and the reemergence of the initial virus.

The authors then investigated if the nAbs recognized the initial or superinfecting virus, and showed that both infections generated unique antibody lineages. Their data also point out that some of the B cell receptors that encode for these nAbs may bind to the transmitted viral envelopes despite not neutralizing the virus.

This study demonstrated that a superinfected individual generated a polyclonal nAb response comprised of antibodies originating from multiple B cell lineages.  One of the lineages was elicited in response to the initial infection, while the others neutralized the subsequent superinfecting virus. The authors reasoned that antibodies that are developed with particular specificities may contribute to plasma breadth, and may provide a deeper understanding into how sequential exposure to diverse HIV strains can influence a polyclonal response and inform vaccine design.  These important findings represent only the second study to isolate mAbs from a superinfected individual.  No doubt, Dr. Overbaugh is already gearing up to start another study.  She enthused: “In the next few weeks, we are starting a study of a second superinfected individual with the same study design as QA013.  We just got NIH funding to continue these studies.  The study of the B cell response in each individual is incredibly labor intensive, but it is important to determine whether the observation that superinfection induces a polyclonal response as a result of different responses to the two infecting viruses is typical.  If so, this would suggest that vaccines designed to mimic superinfection could elicit broad responses that are polyclonal and we think a polyclonal response is likely to be more difficult for the virus to escape compared to a monoclonal response – much like for antiviral drugs, the HIV field found that treatment with one drug was easy to escape; success in treatment came when we started combining drugs.”

Williams KL, Wang B, Arenz D, Williams JA, Dingens AS, Cortez V, Simonich CA, Rainwater S, Lehman DA, Lee KK, Overbaugh J. 2018. Superinfection drives HIV neutralizing antibody responses from several B cell lineages that contribute to a polyclonal repertoire. Cell Reports 23:682—691.

Funding was provided by the National Institutes of Health and the National Science Foundation graduate research fellowship.